This invention relates generally to cable television systems and particularly to such systems for use in connection with stereo sound.
Cable television systems (CATV) are those in which a cable operator, for a fee, supplies a group of subscribers with a converter or decoder for receiving a plurality of television program channels. The term decoder will be used herein to refer to both converters and decoders. All available over-the air commercial television channels are frequency-converted by the operator and supplied to subscribers by cable along with a plurality of other channel signals. Some of the supplementary signals are specially scrambled and require a special decoder (and an additional fee) for viewing. The decoder is under the control of the operator from the so-called cable "head end" and channels may be authorized or nonauthorized by appropriate control signals imposed on the individual channel signal. Thus, the decoder in a viewer's home is, for some purposes, under control of the head end which minimizes its illegal usage as well as precludes reception of channels for which fees have not been received.
Many decoders offer remote control convenience for changing channels, controlling sound levels and muting sound. CATV systems have, in many instances, displaced the tuning and audio control functions of the television receiver to which the CATV decoder is linked since the decoder output is a modulated television signal on the frequency of either VHF channel 3 or 4. Thus the television receiver is "permanently" tuned to channel 3/4 and channel selection is performed at the decoder. Needless to say, remote control is a very desirable feature for viewers.
Recently, stereo sound has been made available for television programming. Television signals that carry stereo sound and also second audio programming (SAP) will do so with additional subcarriers on the 4.5 MHz audio carrier. As this new service grows in popularity, cable operators will want to offer television stereo and SAP to their subscribers. Many present converters have audio circuits that are incapable of properly translating such a stereo/SAP audio signal. Further, many converters detect the 4.5 MHz intercarrier audio signal to produce baseband audio for volume control and mute purposes. Consequently, there is a need in the art for a CATV decoder capable of translating television signals with stereo audio.